Palladium (Pd) is one of the rare metals, and its fine particles are industrially used in various oxidation, reduction, dehydration reaction catalysts, such as catalysts for purification of exhaust gas from automobiles, motorcycles, stationary motors, and factories (three-way catalysts), electrode catalysts for domestic fuel cells in ENE-FARM systems, and VOC removing catalysts. However, fine particles of palladium used in these catalysts are poisoned by CO (carbon monoxide) generated during various chemical reactions, which makes it difficult to use the particles at high power for a long period of time. Thus, there have been many studies on techniques to reduce the degradation of catalysts due to such poisoning. Ruthenium, one of the platinum group metals, is durable against CO poisoning because of its catalytic activity to oxidize CO to CO2 (carbon dioxide). Thus ruthenium has been used in the form of an alloy with other metals, such as platinum, in electrodes of fuel cells in order to reduce CO poisoning. However, palladium and ruthenium are mutually immiscible in equilibrium at the atomic level (i.e., they cannot form a solid solution), and they are separated from each other. A combination of two metals, Rh and Ag or Rh and Au, is also immiscible at the atomic level.
Patent Literature 1 discloses the use of Pd—Ru solid-solution-alloy fine particles as a catalyst, and Patent Literature 2 discloses alloy fine particles in which silver and rhodium or gold and rhodium are present in the form of a solid solution. However, these solid solutions of two metals are thermally unstable, causing concern over durability at high temperatures.